Phosphorylation down-regulates the store-operated Ca2+ entry pathway of human neutrophils

Anchoring protein AKAP79-mediated PKA phosphorylation of STIM1 determines selective activation of the ARC channel, a store-independent Orai channel

KEY POINTS

Although both the calcium store-dependent CRAC channels and the store-independent ARC channels are regulated by the protein STIM1, CRAC channels are regulated by STIM1 in the endoplasmic reticulum, whilst ARC channels are regulated by the STIM1 constitutively resident in the plasma membrane. We now demonstrate that activation of the ARC channels, but not CRAC channels, is uniquely dependent on phosphorylation of a single residue (T389) in the extensive cytosolic domain of STIM1 by protein kinase A. We further demonstrate that the phosphorylation of the T389 residue by protein kinase A is mediated by the association of plasma membrane STIM1 with the scaffolding protein AKAP79. Together, these findings indicate that the phosphorylation status of this single residue in STIM1 represents a key molecular determinant of the relative activities of these two co-existing Ca(2+) entry channels that are known to play critical, but distinct, roles in modulating a variety of physiologically relevant activities.

ABSTRACT

The low-conductance, highly calcium-selective channels encoded by the Orai family of proteins represent a major pathway for the agonist-induced entry of calcium associated with the generation and modulation of the key intracellular calcium signals that initiate and control a wide variety of physiologically important processes in cells. There are two distinct members of this channel family that co-exist endogenously in many cell types: the store-operated Ca(2+) release-activated CRAC channels and the store-independent arachidonic acid-regulated ARC channels. Although the activities of both channels are regulated by the stromal-interacting molecule-1 (STIM1) protein, two distinct pools of this protein are responsible, with the major pool of STIM1 in the endoplasmic reticulum membrane regulating CRAC channel activity, whilst the minor pool of plasma membrane STIM1 regulates ARC channel activity. We now show that a critical feature in determining this selective activation of the two channels is the phosphorylation status of a single threonine residue (T389) within the extensive (∼450 residue) cytosolic domain of STIM1. Specifically, protein kinase A (PKA)-mediated phosphorylation of T389 of STIM1 is necessary for effective activation of the ARC channels, whilst phosphorylation of the same residue actually inhibits the ability of STIM1 to activate the CRAC channels. We further demonstrate that the PKA-mediated phosphorylation of T389 occurs at the plasma membrane via the involvement of the anchoring protein AKAP79, which is constitutively associated with the pool of STIM1 in the plasma membrane. The novel mechanism we have described provides a means for the cell to precisely regulate the relative activities of these two channels to independently modulate the resulting intracellular calcium signals in a physiologically relevant manner.

Tyrosine phosphorylation modulates store-operated calcium entry in cultured rat epididymal basal cells

Store-operated calcium entry (SOCE) is essential for many cellular processes. In this study, we investigated modulation of SOCE by tyrosine phosphorylation in rat epididymal basal cells. The intracellular Ca(2+) ([Ca(2+)]i) measurement showed that SOCE occurred in rat epididymal basal cells by pretreating the cells with thapsigargin (Tg), the inhibitor of sarco-endoplasmic reticulum Ca(2+)-ATPase. To identify the role of Ca(2+) channels in this response, we examined the effects of transient receptor potential canonical channel blockers 2-aminoethoxydiphenyl borate (2-APB), 1-[β-[3-(4-methoxyphenyl)pro-poxy]-4-methoxyphenethyl]-1H-imidazole hydrochloride(SKF96365), Gd(3+), and non-selective cation channel blocker Ni(2+) respectively on SOCE and found that these blockers could inhibit the Ca(2+) influx to different extent. Furthermore, we studied the regulation of SOCE by tyrosine kinase pathway. The inhibitor of tyrosine kinase genistein remarkably suppressed the SOCE response, whereas sodium orthovanadate, the inhibitor of tyrosine phosphatase, greatly enhanced it. The results suggest that tyrosine kinase pathway plays a significant role in the initiation of SOCE and positively modulates SOCE in epididymal basal cells.

Differential effects of intravenous anesthetics on capacitative calcium entry in human pulmonary artery smooth muscle cells

We assessed the roles of the protein kinase C (PKC) and the tyrosine kinase (TK) signaling pathways in regulating capacitative calcium entry (CCE) in human pulmonary artery smooth muscle cells (PASMCs) and investigated the effects of intravenous anesthetics (midazolam, propofol, thiopental, ketamine, etomidate, morphine, and fentanyl) on CCE in human PASMCs. Fura-2-loaded human PASMCs were placed in a dish (37°C) on an inverted fluorescence microscope. Intracellular Ca2+concentration ([Ca2+]i) was measured as the 340/380 fluorescence ratio in individual PASMCs. Thapsigargin, a sarcoplasmic reticulum Ca2+-adenosine triphosphatase inhibitor, was used to deplete intracellular Ca2+stores after removing extracellular Ca2+. CCE was then activated by restoring extracellular Ca2+(2.2 mM). The effects of PKC activation and inhibition, TK inhibition, and the intravenous anesthetics on CCE were assessed. Thapsigargin caused a transient increase in [Ca2+]i. Restoring extracellular Ca2+caused a rapid peak increase in [Ca2+]i, followed by a sustained increase in [Ca2+]i; i.e., CCE was stimulated in human PASMCs. PKC activation attenuated ( P < 0.05), whereas PKC inhibition potentiated ( P < 0.05), both peak and sustained CCE. TK inhibition attenuated ( P < 0.05) both peak and sustained CCE. Midazolam, propofol, and thiopental each attenuated ( P < 0.05) both peak and sustained CCE, whereas ketamine, etomidate, morphine, and fentanyl had no effect on CCE. Our results suggest that CCE in human PASMCs is influenced by both the TK and PKC signaling pathways. Midazolam, propofol, and thiopental each attenuated CCE, whereas ketamine, etomidate, morphine, and fentanyl had no effect on CCE.

The long and arduous road to CRAC

Store-operated calcium (SOC) entry is the major route of calcium influx in non-excitable cells, especially immune cells. The best characterized store-operated current, I(CRAC), is carried by calcium release activated calcium (CRAC) channels. The existence of the phenomenon of store-operated calcium influx was proposed almost two decades ago. However, in spite of rigorous research by many laboratories, the identity of the key molecules participating in the process has remained a mystery. In all these years, multiple different approaches have been adopted by countless researchers to identify the molecular players in this fundamental process. Along the way, many crucial discoveries have been made, some of which have been summarized here. The last couple of years have seen significant breakthroughs in the field-identification of STIM1 as the store Ca(2+) sensor and CRACM1 (Orai1) as the pore-forming subunit of the CRAC channel. The field is now actively engaged in deciphering the gating mechanism of CRAC channels. We summarize here the latest progress in this direction.

Calcium signalling in the endothelium

Elevations in cytosolic Ca2+ concentration are the usual initial response of endothelial cells to hormonal and chemical transmitters and to changes in physical parameters, and many endothelial functions are dependent upon changes in Ca2+ signals produced. Endothelial cell Ca2+ signalling shares similar features with other electrically non-excitable cell types, but has features unique to endothelial cells. This chapter discusses the major components of endothelial cell Ca2+ signalling.

Altered Ca2+ homeostasis in polymorphonuclear leukocytes from chronic myeloid leukaemia patients

Background

In polymorphonuclear leukocytes (PMNL), mobilization of calcium ions is one of the early events triggered by binding of chemoattractant to its receptors. Besides chemotaxis, a variety of other functional responses are dependent on calcium ion mobilization. PMNL from chronic myeloid leukaemia (CML) patients that were morphologically indistinguishable from normal PMNL were found to be defective in various functions stimulated by a chemoattractant – fMLP. To study the mechanism underlying defective functions in CML PMNL, we studied calcium mobilization in CML PMNL in response to two different classical chemoattractants, fMLP and C5a.

Results

Release of calcium estimated by flow cytometry and spectrofluorimetry using fluo-3 as an indicator showed that the [Ca²⁺]_i levels were lower in CML PMNL as compared to those in normal PMNL. But, both normal and CML PMNL showed maximum [Ca²⁺]_i in response to fMLP and C5a at 10 sec and 30 sec, respectively. Spectrofluorimetric analysis of the total calcium release in chemoattractant treated PMNL indicated more and faster efflux of [Ca²⁺]_i in CML PMNL as compared to normal PMNL.

Conclusion

Fine-tuning of Ca²⁺ homeostasis was altered in CML PMNL. The altered Ca²⁺ homeostasis may contribute to the defective functions of CML PMNL.

Selective roles for alpha-PKC in positive signaling for O-(2) generation and calcium mobilization but not elastase release in differentiated HL60 cells

Protein kinase C (PKC) isotypes and Ca2+ mobilization have been implicated in phagocytic cell functions such as O(-)(2) generation. Ca/DG-dependent alpha-PKC and beta-PKC have similar substrate specificities and cofactor requirements in vitro. However it is not known if these isotypes play redundant or unique roles in the intact cell. In the present study, a role for alpha-PKC in positive signaling for fMet-Leu-Phe- and PMA-activated O(-)(2) generation was probed using an siRNA strategy in HL60 cells differentiated to a neutrophilic phenotype (dHL60 cells). A selective decrease in alpha-PKC in dHL60 cells attenuated O(-)(2) generation but not degranulation, and reduced ligand-induced phosphorylation of p47phox as previously shown for beta-PKC. However alpha-PKC, unlike beta-PKC, was a positive regulator of fMet-Leu-Phe-triggered Ca2+ uptake via SOCC (Store Operated Calcium Channels). The ability of a selective SOCC inhibitor, MRS1845, to decrease fMet-Leu-Phe induced Ca2+ uptake and O(-)(2) generation confirmed that Ca2+ uptake via SOCC was required for O(-)(2) generation. These results indicate that alpha-PKC and beta-PKC are required for optimal O(-)(2) generation, but play different roles in Ca2+ signaling for phagocytic responses such as O(-)(2) generation.

Inhibition by calyculin A and okadaic acid of the Ca(2+) release-activated Ca(2+) entry pathway in rat basophilic leukemia cells: evidence for regulation by type 1/2A serine/threonine phosphatase activity

Using a combination of fluorescence measurements of intracellular Ca(2+) ion concentration ([Ca(2+)](i)) and membrane potential we have investigated the sensitivity to serine/threonine phosphatase inhibition of Ca(2+) entry stimulated by activation of the Ca(2+) release-activated Ca(2+) (CRAC) entry pathway in rat basophilic leukemia cells. In both suspension and adherent cells, addition of the type 1/2A phosphatase inhibitor calyculin A, during activation of CRAC uptake, resulted in a fall in [Ca(2+)](i) to near preactivation levels. Pre-treatment with calyculin A abolished the component of the Ca(2+) rise associated with activation of CRAC uptake and inhibited Mn(2+) entry, consistent with a requirement of phosphatase activity for activation of the pathway. Depletion of intracellular Ca(2+) stores is accompanied by a large depolarisation which is absolutely dependent upon Ca(2+) entry via the CRAC uptake pathway. Application of calyculin A or okadaic acid, a structurally unrelated phosphatase antagonist inhibits this depolarisation. Taken in concert, these data demonstrate a marked sensitivity of the CRAC entry pathway to inhibition by calyculin A and okadaic acid.

Distinct effects of N-ethylmaleimide on formyl peptide- and cyclopiazonic acid-induced Ca2+ signals through thiol modification in neutrophils

In this study, we demonstrate that N-ethylmaleimide (NEM), a cell permeable thiol-alkylating agent, enhanced the [Ca2+]i rise caused by stimulation with cyclopiazonic acid (CPA), a sarcoplasmic-endoplasmic reticulum Ca2+-ATPase inhibitor, in rat neutrophils. In addition, NEM attenuated the formyl-Met-Leu-Phe (fMLP)-induced [Ca2+]i rise whether NEM was added to cells prior to or after fMLP stimulation. Moreover, application of NEM after fMLP activation in the absence of external Ca2+ inhibited the Ca2+ signal upon addition of Ca2+ to the medium. Similar patterns were also obtained by using 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB), a cell impermeable dithiol-oxidizing agent, which replaced NEM in the CPA- and fMLP-induced [Ca2+]i rise experiments. Treatment with dithiothreitol (DTT), a cell permeable dithiol-reducing agent, N-acetyl-l-cysteine (NAC), a cell permeable monothiol-reducing agent, and tris-(2-carboxyethyl)phosphine (TCEP), a cell impermeable reductant without a thiol group, all rescued the fMLP-induced Ca2+ signal from NEM. Rat neutrophils express the mRNA encoding for transient receptor potential (TRP) C6, inositol trisphosphate receptor (IP3R) 2 and IP3R3. NEM had no effect on the mitochondrial membrane potential. NEM could restore the polarization and F-actin accumulation of fMLP-treated cells to those of the control. In the absence of external Ca2+, NEM rendered the CPA-induced [Ca2+]i elevation persistently but inhibited the fMLP-induced Ca2+ spike, which was reversed by tris-(2-cyanoethyl)phosphine (TCP), a cell permeable reductant without a thiol group. DTNB did not affect the Ca2+ spike caused by fMLP. These results indicate that through protein thiol oxidation, NEM affects the receptor-activated and the store depletion-derived Ca2+ signals in an opposing manner.

Store-Operated Calcium Channels

In electrically nonexcitable cells, Ca2+influx is essential for regulating a host of kinetically distinct processes involving exocytosis, enzyme control, gene regulation, cell growth and proliferation, and apoptosis. The major Ca2+entry pathway in these cells is the store-operated one, in which the emptying of intracellular Ca2+stores activates Ca2+influx (store-operated Ca2+entry, or capacitative Ca2+entry). Several biophysically distinct store-operated currents have been reported, but the best characterized is the Ca2+release-activated Ca2+current, ICRAC. Although it was initially considered to function only in nonexcitable cells, growing evidence now points towards a central role for ICRAC-like currents in excitable cells too. In spite of intense research, the signal that relays the store Ca2+content to CRAC channels in the plasma membrane, as well as the molecular identity of the Ca2+sensor within the stores, remains elusive. Resolution of these issues would be greatly helped by the identification of the CRAC channel gene. In some systems, evidence suggests that store-operated channels might be related to TRP homologs, although no consensus has yet been reached. Better understood are mechanisms that inactivate store-operated entry and hence control the overall duration of Ca2+entry. Recent work has revealed a central role for mitochondria in the regulation of ICRAC, and this is particularly prominent under physiological conditions. ICRACtherefore represents a dynamic interplay between endoplasmic reticulum, mitochondria, and plasma membrane. In this review, we describe the key electrophysiological features of ICRACand other store-operated Ca2+currents and how they are regulated, and we consider recent advances that have shed insight into the molecular mechanisms involved in this ubiquitous and vital Ca2+entry pathway.

Investigation into the relationship between calyculin A-mediated potentiation of NADPH oxidase activity and inhibition of store-operated uptake of calcium by human neutrophils

The primary objective of the current study was to investigate possible relationships between calyculin A (CA)-mediated potentiation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activity and inhibition of store-operated uptake of Ca2+ by chemoattractant-activated human neutrophils. Treatment of neutrophils with 100 nM CA, but not at lower concentrations (12.5-50 nM), prior to the addition of the N-formylated chemotactic tripeptide, N-formyl-L-methionyl-L-leucyl-L-phenylalanine (FMLP) (1 microM), both potentiated and prolonged the activity of NADPH oxidase which was accompanied by exaggerated membrane depolarisation, delayed and attenuated membrane repolarisation, and inhibition of store-operated Ca2+ influx. Inclusion of diphenylene iodonium chloride (DPI, 10 microM), an inhibitor of NADPH oxidase, antagonised the effects of CA on NADPH oxidase activity and the membrane repolarisation responses of FMLP-activated neutrophils, but failed to restore store-operated influx of Ca2+. Similarly, CA also inhibited store-operated influx of Ca2+ into FMLP-activated neutrophils from a patient with chronic granulomatous disease, a primary immunodeficiency disorder characterised by the absence of a functional NADPH oxidase. CA also inhibited the store-operated influx of Ca2+ into control neutrophils treated with 1 microM thapsigargin, a selective inhibitor of the endomembrane Ca2+-ATPase, which does not activate NADPH oxidase. Taken together, these observations demonstrate that augmentation of NADPH oxidase activity is not primarily involved in CA-mediated inhibition of the store-operated influx of Ca2+ into activated human neutrophils.

Phosphatase Inhibition Reveals a Calcium Entry Pathway Dependent on Protein Kinase A in Thyroid FRTL-5 Cells

Calcium entry through store-operated calcium channels is an important entry mechanism. In the present report we have described a novel calcium entry pathway that is independent of depletion of intracellular calcium stores. Treatment of the cells with the phosphatase inhibitor calyculin A (caly A), which blocked thapsigargin-evoked store-operated calcium entry (SOCE), induced a potent concentration-dependent calcium entry. In a calcium-free buffer, acute addition of caly A evoked a very modest increase in cytosolic free calcium ([Ca(2+)](i)). This increase was not from the agonist-mobilizable calcium stores, as the thapsigargin-evoked increase in [Ca(2+)](i) was unaltered in caly A-treated cells. The caly A-evoked calcium entry was not blocked by Gd(3+) or 2-APB, whereas SOCE was. Caly A enhanced the entry of barium, indicating that the increase in intracellular calcium was not the result of a decreased extrusion of calcium from the cytosol. Jasplakinolide and cytochalasin D had only marginal effects on calcium entry. The protein kinase A (PKA) inhibitor H-89 and an inhibitory peptide for PKA abolished the caly A-evoked entry of both calcium and barium. The SOCE was, however, enhanced in cells treated with H-89. In cells grown in the absence of thyrotropin (TSH), the caly A-evoked entry of calcium was smaller compared with cells grown in TSH-containing buffer. Stimulation of cells grown without TSH with forskolin or TSH restored the calyculin A-evoked calcium entry to that seen in cells grown in TSH-containing buffer. SOCE was decreased in these cells. Our results thus suggest that TSH, through the production of cAMP and activation of PKA, regulates a calcium entry pathway in thyroid cells. The pathway is distinctly different from the SOCE. As TSH is the main regulator of thyroid cells, we suggest that the novel calcium entry pathway participates in the regulation of basal calcium levels in thyroid cells.

Store depletion-induced calcium influx in rat cerebellar astrocytes

1. In rat cerebellar astrocytes, intracellular Ca(2+) store depletion by receptor agonists or sarco(endo)plasmic reticulum Ca(2+) ATPase inhibitors induced a transient increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) in the absence of extracellular Ca(2+) and a sustained increase in its presence. 2. After 10 min treatment with thapsigargin, the [Ca(2+)](i) was unaffected by removal of thapsigargin, but fell rapidly to the basal level when extracellular Ca(2+) was removed, suggesting the involvement of capacitative Ca(2+) entry (CCE); this effect was not seen until cells had been exposed to thapsigargin for at least 2 min. 3. Using the whole cell voltage clamp technique, a 60-100 pA inward current was activated by store depletion, the reversal potential ranging from -5 to 0 mV. 4. When extracellular Na(+) was isotonically replaced by Tris, the thapsigargin-induced [Ca(2+)](i) increase was enhanced, while the inward current was reduced, indicating that store-operated Ca(2+) channels were permeable to Na(+); however, they were not permeable to Sr(2+) or Ba(2+). 5. Thapsigargin-induced CCE remained the same in the presence of nifedipine, La(3+) or Cd(2+), while it was inhibited in the presence of SK&F96365. 6. In cerebellar astrocytes, inhibition of protein serine/threonine phosphorylation promoted CCE. 7. In conclusion, in rat cerebellar astrocytes, store depletion activated a CCE via channels which were permeable to Ca(2+) and Na(+) and regulated by phosphorylation.

Cellular mechanisms of inhibition of superoxide anion generation in rat neutrophils by the synthetic isoquinoline DMDI

This study was undertaken to assess the cellular localization of the inhibitory effect of a chemically synthetic isoquinoline compound 1-(3',4'-dimethoxybenzyl)-6,7-dichloroisoquinoline (DMDI) on the formyl-methionyl-leucyl-phenylalanine (fMLP)-induced respiratory burst in rat neutrophils. The DMDI concentration dependently inhibited the superoxide anion (O(2)(*-)) generation and O(2) consumption (IC(50) 12.2+/-4.9 and 15.2+/-8.4 microM, respectively) of neutrophils. DMDI did not scavenge the O(2)(*-) generated during the autoxidation of dihydroxyfumaric acid in a cell-free system. DMDI did not elevate cellular cyclic AMP levels. Inhibition of O(2)(*-) generation by DMDI in neutrophils was not reversed by a cyclic AMP-dependent protein kinase inhibitor, (8R,9S,11S)-(-)-9-hydroxy-9-hexoxycarbonyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-triazadibenzo[a,g]cycloocta[cde]trinden-1-one (KT5720). The DMDI concentration dependently inhibited the late plateau phase but not the initial spike of fMLP-induced [Ca(2+)](i) changes in the presence of extracellular Ca(2+). However, DMDI had no effect on the fMLP-induced [Ca(2+)](i) changes in the absence of extracellular Ca(2+). In addition, DMDI did not affect the fMLP-stimulated phosphatidylinositol 3-kinase (PI3-kinase) activation. DMDI produced a concentration-dependent reduction in the formation of phosphatidic acid and phosphatidylethanol in the presence of ethanol from fMLP-stimulated neutrophils (IC(50) 13.3+/-4.0 and 9.4+/-4.3 microM, respectively). On the basis of the immunoblot analysis of the phosphorylation of the mitogen-activated protein (MAP) kinase, DMDI attenuated the fMLP-stimulated MAP kinase phosphorylation in a similar concentration range. Collectively, these results indicate that the inhibition of the respiratory burst by DMDI in rat neutrophils is mediated through the blockade of phospholipase D and MAP kinase signaling pathways.

Inhibition of extracellular Ca(2+) entry by YC-1, an activator of soluble guanylyl cyclase, through a cyclic GMP-independent pathway in rat neutrophils

The effects of a soluble guanylyl cyclase (sGC) activator, 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole (YC-1), on formyl-methionyl-leucyl-phenylalanine (fMLP)-stimulated [Ca(2+)](i) elevation in rat neutrophils were examined. YC-1 produced a concentration-dependent inhibition of [Ca(2+)](i) elevation. Pretreatment of neutrophils with YC-1 did not enhance its inhibitory effect. YC-1 also inhibited the [Ca(2+)](i) changes caused by ionomycin. In a biphasic model, measuring the [Ca(2+)](i) stimulation by fMLP in a Ca(2+)-free medium followed by reintroduction of Ca(2+), YC-1 mainly affected Ca(2+) influx. YC-1 also inhibited active and passive Mn(2+) influx, and this inhibitory effect was not attenuated by the sGC inhibitor 6-anilino-5,8-quinolinequinone (LY83583). Sodium nitroprusside did not affect the fMLP-stimulated [Ca(2+)](i) changes. Pretreatment of neutrophils with the cyclic GMP-dependent protein kinase inhibitor 8-(4-chlorophenylthio) guanosine-3',5'-monophosphorothioate, Rp-isomer (Rp-8-pCPT-cGMPS), LY83583, the protein phosphatase 2B inhibitor cyclosporin A, or the protein kinase inhibitor staurosporine did not attenuate the inhibition of [Ca(2+)](i) by YC-1. YC-1 inhibited the fMLP-stimulated protein tyrosine phosphorylation. These results indicate that cyclic GMP does not play an important role in the regulation of [Ca(2+)](i) in rat neutrophils. Inhibition of fMLP-stimulated [Ca(2+)](i) changes by YC-1 is mainly via the blockade of Ca(2+) entry through the inhibition of tyrosine kinase activity, but not the stimulation of protein kinase C and protein phosphatase 2B.

Negative regulation of ligand-initiated Ca(2+) uptake by PKC-beta II in differentiated HL60 cells

In phagocytic cells, fMet-Leu-Phe triggers phosphoinositide remodeling, activation of protein kinase C (PKC), release of intracellular Ca(2+) and uptake of extracellular Ca(2+). Uptake of extracellular Ca(2+) can be triggered by store-operated Ca(2+) channels (SOCC) and via a receptor-operated nonselective cation channel(s). In neutrophilic HL60 cells, the PKC activator phorbol myristate acetate (PMA) activates multiple PKC isotypes, PKC-alpha, PKC-beta, and PKC-delta, and inhibits ligand-initiated mobilization of intracellular Ca(2+) and uptake of extracellular Ca(2+). Therefore PKC is a negative regulator at several points in Ca(2+) mobilization. In contrast, selective depletion of PKC-beta in HL60 cells by an antisense strategy enhanced fMet-Leu-Phe-initiated Ca(2+) uptake but not mobilization of intracellular Ca(2+). Thapsigargin-induced Ca(2+) uptake through SOCC was not affected by PKC-beta II depletion. Thus PKC-beta II is a selective negative regulator of Ca(2+) uptake but not release of intracellular Ca(2+) stores. PKC-beta II inhibits a receptor-operated cation or Ca(2+) channel, thus inhibiting ligand-initiated Ca(2+) uptake.

Chlorpromazine inhibits store-operated calcium entry and subsequent noradrenaline secretion in PC12 cells

1. The effect of chlorpromazine on the store-operated Ca2+ entry activated via the phospholipase C signalling pathway was investigated in PC12 cells. 2. Chlorpromazine inhibited the sustained increase after the initial peak in the intracellular Ca2+ concentration produced by bradykinin while having no effect on the initial transient response. The inhibition was lowered by the removal of extracellular free Ca2+. However, chlorpromazine did not inhibit bradykinin-induced inositol 1,4,5-trisphosphate production. 3. Chlorpromazine inhibited the bradykinin-induced noradrenaline secretion in a concentration-dependent manner (IC(50): 24+/-5 microM, n=3). 4. To test for a direct effect of chlorpromazine on store-operated Ca2+ entry, thapsigargin, an inhibitor of microsomal Ca(2+)-ATPase, was used to induce store-operated Ca2+ entry in PC12 cells. Chlorpromazine reduced the thapsigargin-induced sustained Ca2+ level (IC(50): 24+/-2 microM, n=3), and the inhibition also occluded the inhibitory action of 1-[-[3-(4-methoxyphenyl) propoxy]-4-methoxyphenyl]-1H-imidazole hydrochloride (SK&F96365). 5. The results suggest that chlorpromazine negatively modulates the store-operated Ca2+ entry activated subsequent to PLC activation.

Protein kinase C potentiates capacitative Ca2+ entry that links to steroidogenesis in bovine adrenocortical cells

I investigated the role of protein kinase C (PKC) in regulation of the capacitative Ca2+ entry and steroidogenesis in bovine adrenocortical (BA) cells. Thapsigargin (TG)-treatment depleted intracellular Ca2+ stores followed by induction of Ca2+ influx from the extracellular pool and also increasing of Mn2+ influx as an indicator of divalent cation influx in BA cells. Calphostin C, a PKC inhibitor, inhibited the TG-induced [Ca2+]i elevation dose-dependently (0.1-1 microM) and attenuated Mn2+ entry. Phorbol 12-myristate 13-acetate (PMA), an activator of PKC, potentiated the elevation of [Ca2+]i and enhanced Mn2+ entry by TG treatment. These results suggest that PKC may modulate capacitative Ca2+ entry in BA cells. In the presence of extracellular Ca2+, TG enhanced cortisol production in BA cells. Calphostin C attenuated the TG-induced steroidogenesis dose-dependently (0.25-1 microM). PMA enhanced the steroidogenesis dose-dependently (1-100 nM). These results suggested that PKC may have a modulatory effect on the capacitative Ca2+ entry that links to steroidogenesis in BA cells.

Phenylarsine oxide increases intracellular calcium mobility and inhibits Ca(2+)-dependent ATPase activity in thymocytes

A rise in intracellular Ca(2+) levels has been implicated as a regulatory signal for the initiation of lymphocyte proliferation. In the present study the mechanism underlying the elevation of [Ca(2+)] induced by phenylarsine oxide [PAO] was investigated in thymocytes. This agent inhibits HIV-1 replication and also NF-kappaB-mediated activation. It has been reported that the PAO-induced Ca(2+) elevation results from an enhanced plasma membrane calcium permeability in T cells. Here, we present biochemical evidence that the PAO-induced Ca(2+) increase was independent of external Ca(2+). Consistent with these facts, when [Ca(2+)](i) was depleted by prolonged incubation of the cells in Ca(2+)-free medium, PAO addition did not lead to [Ca(2+)](i) increase. These data indicate the involvement of intracellular organelles of thymocytes as the source of Ca(2+). Moreover, evidence is presented that PAO inhibited Ca(2+)-dependent ATPase activity from thymocytes and sarcoplasmic reticulum from skeletal muscle. This inhibition was dose-dependent, with a IC(50) of about 30 microM for both preparations of enzyme. The ability of PAO to inhibit Ca(2+)-dependent ATPase represents a novel mechanism of action for this drug. Present data suggest that the PAO-dependent [Ca(2+)](i) increase could be mainly the result of inhibition of Ca(2+)-dependent ATPase. In addition, we describe also a Ca(2+)-dependence for PAO effect on tyrosine phosphorylation.

Opposing effects of protein kinase A and C on capacitative calcium entry into HL-60 promyelocytes

Treatment of HL-60 cells with thapsigargin, a microsomal Ca2+/ATPase inhibitor, led to depletion of intracellular calcium stores followed by capacitative calcium entry. Stimulation of adenylyl cyclase with forskolin enhanced thapsigargin-induced Ca2+ influx. The forskolin effect was confirmed by enhanced fluorescence quenching induced by Mn2+ entry into fura-2 loaded cells. 1,9-Dideoxy-forskolin, an inactive analog of forskolin, did not affect capacitative calcium entry. On the other hand, phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C, inhibited thapsigargin-induced Ca2+ entry. Histamine and prostaglandin E2 (PGE2) elevated intracellular adenosine 3':5'-cyclic monophosphate (cAMP) levels and enhanced the thapsigargin-induced capacitative calcium entry. Incubation with N-[2-(p-bromocynnamylamino)ethyl]-5-isoquinolinesulfonamide (H89), an inhibitor of protein kinase A (PKA), blocked the forskolin effect, and GF109203X, an inhibitor of protein kinase C (PKC), blocked the phorbol 12-myristate 13-acetate effect. The results suggest that protein kinase A regulates capacitative calcium entry positively, but that protein kinase C regulates Ca2+ influx negatively. Furthermore, after differentiation of HL-60 promyelocytes with dimethylsulfoxide to granulocytes, the inhibitory effect of phorbol 12-myristate 13-acetate became more pronounced, whereas the stimulatory effect of prostaglandin E2 did not change. This result suggests that the regulation of capacitative calcium entry by protein kinase C and protein kinase A develops differently during differentiation.

Receptor-operated calcium influx mediated by protein tyrosine kinase pathways

Calcium influx from the extracellular space elicited by activation of heterotrimeric G protein-coupled and heptahelical receptors plays a critical role in transmembrane signal transduction in a wide variety of cell systems. In nonexcitable cells, the precise voltage-independent mechanism by which calcium enters the cell remains unknown. Multiple mechanisms appear to be operating in different cell types (1-3): 1. G protein-operated calcium influx, 2. Second messenger-operated calcium influx, 3. Capacitative calcium influx, and 4. Phosphorylation of calcium channels. Receptor-operated calcium channels have a fundamental role in stimulus-secretion coupling in many different cells, but these channels remain to be purified and cloned. This review proposes that receptor-operated calcium influx is mediated by protein tyrosine kinase pathways. The function of protein tyrosine kinase pathways and their interactions with other receptor-operated calcium influx mechanisms are described.

Regulation of capacitative Ca2+ influx in human neutrophil granulocytes. Alterations in chronic granulomatous disease

Ca2+ entry through the capacitative (store-regulated) pathway was shown to be inhibited in neutrophil granulocytes by the protein kinase C activator phorbol 12-myristate 13-acetate and the chemoattractant N-formyl-methionyl-leucyl-phenylalanine (fMLP) by a hitherto unknown mechanism. Measuring both Ca2+ and Mn2+ entry into store-depleted cells we show in the present study that inhibition of the capacitative pathway is absent in various forms of chronic granulomatous disease. To establish the possible relationship between inhibition of the capacitative pathway and ability of O-2 production and consequent membrane depolarization, gradual changes of the membrane potential were evoked in neutrophils of healthy individuals. This was accomplished by pharmacological manipulation of the membrane potential and by variations of the concentration and type of the stimulant. Close relationship was observed between membrane depolarization and inhibition of Mn2+ entry through the capacitative transport route. Our results provide an explanation for the inhibitory action of fMLP and phorbol 12-myristate 13-acetate on capacitative cation influx and reveal that upon physiological stimulation, Ca2+ entry into neutrophils is restricted by the depolarization accompanying O-2 production.

Arachidonic acid inhibits capacitative calcium entry in rat thymocytes and human neutrophils

Emptying the intracellular Ca2+ stores by treatment with the endomembrane Ca2+-ATPase inhibitor thapsigargin activates capacitative Ca2+ entry (CCE). This can be evidenced in fura-2-loaded cells by an increase of [Ca2+]i or by an acceleration of Mn2+ entry. Micromolar concentrations of arachidonic acid inhibited CCE induced by treatment with thapsigargin in rat thymocytes and in human neutrophils. This inhibitory action was shared by other unsaturated fatty acids, but not by the saturated arachidic acid nor by arachidonic acid methyl ester. The effect was not due to metabolites derived from arachidonic acid since several non-metabolizable analogs were able to reproduce it. Phorbol dibutyrate (PDB) acted similarly, suggesting that the inhibitory effect could be mediated by activation of protein kinase C (PKC). However, whereas the inhibition of CCE by PDB was reversed by treatment with the PKC inhibitor staurosporin, the inhibition by arachidonic acid was not. We find that unsaturated fatty acids antagonized microsomal dealkylation of benzyl-resorufin, a cytochrome P450-mediated activity, with the same specificity profile as for inhibition of CCE. These results are consistent with previous proposals suggesting that a microsomal cytochrome P450 may be involved in the regulation of CCE.

Altered intracellular calcium signalling after PAF stimulations in polymorphonuclear leukocytes from asthmatic patients

Platelet activating factor (PAF), a potent lipid mediator, has been implicated in the pathogenesis of airways inflammation in bronchial asthma. Binding of PAF to its receptor (Nakamura et al., 1991) leads to changes of intracellular Ca2+ concentration ([Ca2+]i) that is crucial to cell activation. Therefore, the aim of our study was to investigate whether PMNL of asthmatic patients stimulated with PAF (10(-7) M) differ in relation to changes of [Ca2+]i from cells of healthy subjects. PMNL from asthmatic patients revealed attenuated first response to PAF stimulation--increase in [Ca2+]i (delta[Ca2+]i) was 1.3-fold lower in cells of asthmatics (P < 0.05) versus PMNL of healthy subjects. As determined in experiments with low extracellular calcium concentration, Ca2+ release from internal stores tended to be increased in asthmatics and hence the difference in total Ca2+ response was related to decrease in Ca2+ influx. Thus the contribution of Ca2+ from internal stores to the total first Ca2+ response upon PAF stimulation was two-fold higher (58 +/- 18 vs. 29 +/- 8%, P < 0.001) in PMNL of asthmatics compared with healthy subjects. Two subsequent Ca2+ responses evoked by stimulations with the agonist in 1 mM Ca2+ buffer did not differ between study groups. In low Ca2+ buffer PMNL of 50% of asthmatics responded to the second stimulation while cells of healthy subjects remained unresponsive. The altered Ca2+ responses in PMNL of asthmatic subjects may reflect previous contact with mediator(s) that occur in vivo which may be at least partially explained by the phenomenon of down regulation reported for PAF receptor upon cell stimulation.

A role for protein kinase CbetaI in the regulation of Ca2+ entry in Jurkat T cells

T cell activation leading to cytokine production and cellular proliferation involves a regulated increase and subsequent decrease in the intracellular concentration of Ca2+ ([Ca2+]i). While much is understood about agonist-induced increases in [Ca2+]i, less is known about down-regulation of this pathway. Understanding the mechanism of this down-regulation is critical to the prevention of cell death that can be the consequence of a sustained elevation in [Ca2+]i. Protein kinase C (PKC), activated by the diacylglycerol produced as a consequence of T cell receptor engagement, has long been presumed to be involved in this down-regulation, although the precise mechanism is not wholly clear. In this report we demonstrate that activation of PKC by phorbol esters slightly decreases the rate of Ca2+ efflux from the cytosol of Jurkat T cells following stimulation through the T cell receptor or stimulation in a receptor-independent manner by thapsigargin. On the other hand, phorbol ester treatment dramatically reduces the rate of Ca2+ influx following stimulation. Phorbol ester treatment is without an effect on Ca2+ influx in a different T cell line, HSB. Down-regulation of PKCbetaI expression by 18-h phorbol ester treatment is associated with a loss of the response to acute phorbol ester treatment in Jurkat cells, suggesting that PKCbetaI may be the isozyme responsible for the effects on Ca2+ influx. Electroporation of an anti-PKCbetaI antibody, but not antibodies against PKCalpha or PKCgamma, led to an increase in the rate of Ca2+ influx following stimulation. Taken together, these data suggest that PKCbetaI may be a component of the down-regulation of increases in [Ca2+]i associated with Jurkat T cell activation.

Inhibition of store-dependent capacitative Ca2+ influx by unsaturated fatty acids

The effects of the unsaturated fatty acids, arachidonic and oleic acid, on the influx of Ca2+ activated by depletion of intracellular stores with thapsigargin were investigated in various cell types. By using a Ca2+ free/Ca2+ reintroduction protocol, we observed that arachidonic acid (2 to 5 microM) inhibited thapsigargin-induced rises in cytosolic free Ca2+ ([Ca2+]i) in Ehrlich tumor cells, Jurkat T lymphocytes, rat thymocytes, and Friend erythroleukemia and PC12 rat pheochromocytoma cells. This effect was attributed to the inhibition of Ca2+ entry, since arachidonate also inhibited thapsigargin-stimulated unidirectional entry of the Ca2+ surrogates Ba2+ and Mn2+. In Ehrlich cells, the IC50 for arachidonic and oleic acid was 1.2 and 1.8 microM, respectively. The inhibition appeared to depend on the ratio [fatty acid]/[cells] rather than on the absolute fatty acid concentration. Experiments with [3H]-oleic acid revealed that the inhibitory activity was not correlated with cell internalisation and metabolism of the fatty acid. The inhibition was reverted by removal of the fatty acid bound to cell membrane by fatty acid-free albumin treatment. The unsaturated fatty acids had no effect on ATP/ADP cell levels and plasma membrane potential. Pharmacological evidence indicated that cell phosphorylation/dephosphorylation events, and pertussis toxin-sensitive G proteins were not involved. Other amphipathic lipophilic compounds, i.e. 2-bromopalmitic acid, retinoic acid, sphingosine, and dihydrosphingosine, mimicked arachidonic/oleic acid as they inhibited thapsigargin-stimulated Ca2+ influx in an albumin-reversible fashion. These results suggest that physiologically relevant (unsaturated) fatty acids can inhibit capacitative Ca2+ influx possibly because they intercalate into the plasma membrane and directly affect the activity of the channels involved.

Histamine inhibits ATP-induced [Ca2+]i rise through the activation of protein kinase A in HL-60 cells

We investigated the cross-talk between the histamine and ATP receptors in HL-60 human promyelocytes. While both histamine and extracellular ATP increase intracellular Ca2+ concentration ([Ca2+]i) we found that histamine treatment causes a decrease in the subsequent ATP-induced Ca2+ release from intracellular stores and Ca2+ influx from extracellular space. In addition, histamine also inhibited the subsequent ATP-induced inositol 1.4,5-trisphosphate (IP3) generation in a manner comparable to the Ca2+ release. However, histamine did not inhibit thapsigargin-induced Ca2+ release and influx, thus indicating that histamine does not directly inhibit the Ca2+ release-activated channel (CRAC). Ca2+ elevation induced by 2'- and 3'-O-(4-benzoylbenzoyl) ATP (BzATP), which does not produce IP3, was also inhibited by treatment with histamine, suggesting the presence of ATP-gated channels that are regulated by histamine. Treatment with dibutyryl cAMP or 8-bromo-cAMP inhibited the subsequent ATP-induced response similar to histamine. Moreover, the incubation of cells with N-[2-(p-bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide (H89), a protein kinase A inhibitor abolished histamine's inhibitory effect on the ATP-induced [Ca2+]i rise and IP3 formation. These results suggest that histamine inhibits both ATP-induced IP3 production and ATP-activated channel opening, through protein kinase A activation.

Evidence for a phorbol ester-insensitive phosphorylation step in capacitative calcium entry in rat thymic lymphocytes

Experiments were undertaken to investigate the regulation of capacitative Ca2+ entry by phorbol ester-sensitive protein kinase C and serine/threonine protein phosphatase activity. The thapsigargin-activated Ca2+ entry pathway was probed in control cells and cells treated with phosphatase type 1/2A inhibitors, okadaic acid and calyculin A, or with the phorbol ester, phorbol 12-myristate 13-acetate. The permeability state of this pathway was monitored in the presence or absence of these agents using fluorometric measurements of intracellular Ca2+ concentration, unidirectional Mn2+ entry, and membrane potential and unidirectional measurements of Ca2+ uptake using 45Ca2+. The results of these studies demonstrate that modification of the phosphorylation state of target protein(s) on serine/threonine amino acid residues by inhibition of phosphatase type 1/2A inhibits the capacitative Ca2+ entry pathway in rat thymic lymphocytes. Importantly, the capacitative Ca2+ entry pathway in rat thymic lymphocytes is not modulated by activation of phorbol ester-sensitive protein kinase C.

A capacitative calcium current in cultured skeletal muscle cells is mediated by the calcium-specific leak channel and inhibited by dihydropyridine compounds

Calcium stores from cultured skeletal muscle cells were depleted using cyclopiazonic acid (CPA), a reversible inhibitor of Ca2+-ATPases at the sarcoplasmic reticulum. Store depletion led to activation of the calcium-specific leak channel, as assayed using single-channel patch clamp analysis and rates of manganese influx and quenching of fura-2 fluorescence. Two novel dihydropyridine compounds inhibited this single-channel leak channel activity, the resting and depletion-induced manganese influx, and refilling of the CPA-depleted intracellular calcium store. These compounds represent the first antagonists for a calcium leak channel and for a channel that mediates a capacitative current. The development of the skeletal muscle capacitative current was inhibited by genistein, a tyrosine kinase inhibitor, but was not affected by okadaic acid, a phosphatase inhibitor, or econazole. Thus, the capacitative current in cultured skeletal muscle cells was mediated by the calcium leak channel and was inhibited by pharmacological antagonists and may provide a model system for uncovering the complete set of signals leading from store depletion to channel activation.

Okadaic acid-like toxin in systemic lupus erythematosus patients: hypothesis for toxin-induced pathology, immune dysregulation, and transactivation of herpesviruses

Preliminary evidence suggests there is a toxin in the sera of systemic lupus erythematosus patients which reacts with a commercial enzyme-linked immunosorbent assay kit for the detection of the marine toxin, okadaic acid. Data is presented which supports the hypothesis that an okadaic acid-like toxin may be the principle agent of lymphocyte dysregulation in systemic lupus erythematosus and other immune-dysregulated states. The okadaic acid-like toxin can produce the specific abnormalities in T-lymphocyte phenotype and function typical of systemic lupus erythematosus, principally through its ability to inhibit serine/threonine phosphatases necessary for secondary signalling processes and through its ability to inhibit calcium which is crucial to protein kinase C-mediated signalling of T-lymphocytes. The disruption probably occurs through the protein tyrosine kinase p56lck pathway crucial for IL-2. Additionally, the toxin's ability to disrupt voltage-sensitive ion channels in cell membranes may be responsible for the multi-organ pathology observed in systemic lupus erythematosus patients, particularly neurological, cardiac and nephritic. Data from a different study conducted by the author suggests that latent and persistent viruses are reactivated in active lupus. This activation could be the result of the toxin's ability to act as an immune modulator, or its ability to act as a transactivating factor.

Differential effects of protein kinase C activation on calcium storage and capacitative calcium entry in NIH 3T3 cells

In NIH 3T3 cells, treatment with phorbol 12-myristate 13-acetate (PMA) reduced the release of Ca2+ by thapsigargin, but did not activate Ca2+ entry; Ca2+ influx was triggered after the residual pool was emptied by thapsigargin, and this Ca2+ influx was similar to that induced by thapsigargin in control cells. The effect of PMA was due to decreased Ca2+ storage because 1) Ca2+ release by ionomycin was similarly affected by PMA, and in both control and PMA-treated cells, ionomycin did not release Ca2+ following thapsigargin treatment; 2) PMA reduced 45Ca2+ accumulation; and 3) studies with Ca2+ indicator compartmentalized into the endoplasmic reticulum indicated that stored Ca2+ was reduced by PMA. Although PMA did not itself activate Ca2+ entry, PMA potentiated Ca2+ entry with low concentrations of cyclopiazonic acid. With a somewhat higher concentration of cyclopiazonic acid, PMA had no effect on calcium entry. Thus, protein kinase C has two apparent actions on calcium signaling in NIH 3T3 cells: 1) reduced intracellular Ca2+ storage capacity and 2) augmented calcium entry with submaximal intracellular Ca2+ pool depletion. These actions indicate a complex and potentially important role for the protein kinase C system in calcium homeostasis in this cell type.

Pharmacological and functional properties of voltage-independent Ca2+ channels

During the last few years, considerable progress has taken place in our knowledge of the molecular and functional properties of the various voltage-independent Ca2+ channels. In addition to the ionotropic receptor-channels (ROCs), that are not discussed in the present review, these channels include the SMOCs, activated via second messengers or other transducing processes directly triggered by receptor activation; and the SOCCs, activated as a consequence of depletion of the rapidly exchanging Ca2+ stores in the cytoplasm. In parallel, a pharmacological approach to the study of these channels has been developed, based primarily on heterogeneous drugs already known for different biological effects, and subsequently recognized as voltage-independent Ca(2+)-channel blockers. From the systematic analysis of the effects of these drugs new information has emerged about SMOCs and SOCCs function. In addition, pharmacological blockade of these channels appears to have beneficial therapeutic effects in pathological conditions such as tumoral cell growth, inflammation and immunity. At the moment the field is rapidly evolving, with major developments expected in the years ahead.

Suppression of capacitative Ca2+ entry by serine/threonine phosphatase inhibitors in rat parotid acinar cells

The effects of three serine/threonine protein phosphatase inhibitors, calyculin-A, tautomycin and okadaic acid, on the Ca2+ entry across the plasma membrane was studied in Fura-2-loaded rat parotid acinar cells. These protein phosphatase inhibitors did not affect the peak elevation of cytosolic free Ca2+ concentration ([Ca2+]i) just after stimulation with the muscarinic agonist carbachol (CCh), but they suppressed the sustained increase in [Ca2+]i. In the absence of extracellular Ca2+, CCh produced a transient increase in [Ca2+]i due to Ca2+ release from intracellular Ca2+ stores, and this increase in [Ca2+]i was unaffected by the phosphatase inhibitors. When Ca2+ was added to the external medium after the transient [Ca2+]i response, the increase in [Ca2+]i in the cells treated with the phosphatase inhibitors was significantly smaller than that in the control cells, indicating that the Ca2+ entry was reduced. Similar suppression of Ca2+ entry by the phosphatase inhibitors was observed when intracellular Ca2+ stores were previously depleted by the microsomal Ca(2+)-ATPase inhibitor thapsigargin (TG). In addition, the phosphatase inhibitors reduced the Mn2+ (Ca2+ surrogate) influx following the addition of CCh or TG. The enhancement of Ca2+ entry by the protein kinase inhibitor staurosporine was significantly attenuated by the phosphatase inhibitors. These results suggest that the phosphatase inhibitors suppressed the Ca2+ entry mechanism activated by depletion of intracellular Ca2+ stores in rat parotid acinar cells. The capacitative Ca2+ entry may be regulated by protein phosphorylation/dephosphorylation.

Calcium signaling in endothelial cells involves activation of tyrosine kinases and leads to activation of mitogen-activated protein kinases

The activation of endothelial cells following exposure to a variety of receptor-dependent and -independent stimuli is associated with the release of Ca2+ from intracellular stores as well as the influx of Ca2+ from the extracellular space. In the present study, we investigated the interaction between Ca2+ signaling in cultured human umbilical vein endothelial cells and tyrosine phosphorylation. Stimulation of endothelial cells with either bradykinin (100 nmol/L), histamine (1 mumol/L), or the Ca(2+)-ATPase inhibitor thapsigargin (30 nmol/L) resulted in a slightly delayed but prolonged tyrosine phosphorylation of two low molecular weight proteins (approximately 42 and approximately 44 kD). These proteins were identified by immunoprecipitation as the 42- and 44-kD isoforms of mitogen-activated protein kinase (MAP kinase). The agonist-induced tyrosine phosphorylation of the 42-/44-kD doublet was sensitive to the tyrosine kinase inhibitors genistein (100 mumol/L) and piceatannol (10 mumol/L) and was inhibited by the removal of Ca2+ from the extracellular medium. In fura 2-loaded endothelial cells, inhibition of tyrosine kinases attenuated Ca2+ signaling after stimulation with either bradykinin (30 nmol/L) or thapsigargin (30 nmol/L). Since inhibition of tyrosine kinases specifically attenuates the plateau phase of the Ca2+ response after stimulation, the effect of tyrosine kinase inhibition appeared to be mostly associated with the influx of Ca2+ from the extracellular space.(ABSTRACT TRUNCATED AT 250 WORDS)

N-acetylsphingosine (C2-ceramide) inhibited neutrophil superoxide formation and calcium influx

Ceramide, a product arising from sphingomyelinase activity, has been shown to act as an intracellular second messenger in effecting growth inhibition, cellular differentiation, and apoptosis. In the present study, the relative effects of cell-permeable ceramides, N-acetylsphingosine (C2-ceramide) and N-hexanoylsphingosine (C6-ceramide), on neutrophil responses were measured. When cells were activated with fMet-Leu-Phe, C2-ceramide both potentiated (< 1 microM) and inhibited (> 1 microM) superoxide generation. C2- and C6-ceramide inhibited phorbol ester-induced superoxide release from neutrophils at IC50 values of 5 and 120 microM, respectively. C2-ceramide had no effect on semipurified protein kinase C activity. Neither ceramide affected significantly the general level of phosphorylated proteins in phorbol ester-treated cells. C2-ceramide (1-20 microM) alone did not change cytosolic free Ca2+ levels but inhibited Ca2+ and Mn2+ influx in fMet-Leu-Phe-activated neutrophils. In contrast, sphingosine enhanced Ca2+ entry; thus, ceramide conversion to sphingosine was not significant. Unlike C2-ceramide, C2-dihydroceramide failed to block superoxide generation or Ca2+ influx. Preincubation of cells with 10 nM okadaic acid reversed slightly the effects of C2-ceramide. Calyculin A, tautomycin, and much higher concentrations of okadaic acid inhibited agonist-induced Ca2+ influx. We postulate that C2-ceramide may inhibit neutrophil superoxide release by activation of type 2A protein phosphatases. Results suggest that protein phosphatase type 1 up-regulates Ca2+ entry, whereas type 2A (or a ceramide-activated subtype) forestalls Ca2+ entry by inactivating a calcium influx factor.